Electric power is often transformed from high voltage (hundreds of kilovolts) ac to high voltage (hundreds of kilovolts) dc in order to achieve more efficient long distance transmission of electric power and to achieve other network-related operational advantages. In such cases it is then transformed back to high voltage ac at the receiving substation where it is subsequently coupled by transformer into the ac grid. Usefulness of high voltage dc is limited largely by the cost of the ac-to-dc and dc-to-ac converter stations. The cost of those stations benefit from economies of scale, the lowest cost per kilowatt is achieved by stations designed for very high power levels at very high voltages. Because stations which convert small amounts of power at very high voltage are extremely expensive, it is impractical to “tap” long distance high voltage dc lines to supply small intermediate loads.
Unlike transformation between ac and dc at household voltages where insulation between circuit components is a minor cost and rating-availability consideration, insulation of components within equipment designed for hundreds of kilovolts, both between components and from components to ground is a major expense and rating constraint; thus the need for a new approach adaptable to very high voltage equipment and supporting insulation.